This invention relates to a technique for suppressing arcs in an electromagnetic waveguide, and more particularly to a technique that detects the arcs and suppresses them using forced air.
Waveguides have been used for some time as an efficient way to carry microwave frequency energy over distances in a predictable manner. However, waveguides in some instances have a tendency to experience unpredictable behavior such as internal arcing. In particular, even though a waveguide is sized to be capable of operating safely at the expected power levels without introducing a voltage breakdown, certain events or faults may occur to cause an energy discharge within the waveguide itself. Such faults may happen when dust, dirt or other ambient conditions introduce an abnormal voltage condition inside the waveguide. Such arcing may actually continue after the fault is no longer in existence. An arc is of concern because it not only substantially blocks transmission of energy through the waveguide, but also may physically damage the system components.
For example, electromagnetic energy normally travels within the waveguide from an electromagnetic energy source through the waveguide toward a system that makes use of the microwave energy, such as a microwave oven cavity. Once an arc occurs, electromagnetic forces tend to cause it to travel in a reverse direction within the waveguide, back toward the power source. The arc typically absorbs almost half of the forward power, and reflects a similar amount of electromagnetic energy back to the power source. This causes a decrease in power levels at points in the waveguide beyond the arc to negligible levels.
A number of methods have been used in the past to detect and deal with the occurrence of an arc within a waveguide. For example, detectors may be attached to the waveguide which are responsive to the vibratory and electromagnetic disturbances resulting from the arc. The detectors can be arranged not only to determine the existence of an arc but also its location and velocity.
Upon detection of an arc, electronic control circuits can then be used to temporarily shut off the microwave power source or reduce its level so that the arcing will eventually cease. After a suitable delay, to allow any ionization caused by the arc within the waveguide to dissipate, the power source is then brought back on line again.
Arcing can be especially problematic in certain end uses such as microwave ovens. For example, in industrial process type microwave ovens that are used in large scale cooking applications, continuous and predictable microwave energy levels are required to produce a predicable end result of the cooking process. Any need to shut down the oven to extinguish an arc can therefore be very undesirable.
In accordance with one embodiment of the invention, an arc suppression system is provided which includes a waveguide run for carrying microwave energy, a sensing device, such as a photodetector, for sensing an arc within the waveguide run, and a blowing device for blowing a gas, such as compressed air, into the waveguide run, in response to a sensed arc to suppress the arc.
A controller can be connected to the sensing device and the blowing device for opening a valve of the blowing device, in response to the sensed arc, to allow the gas to suppress the sensed arc. A second blowing device can also be provided for blowing a gas to clean a viewing surface of the sensing device.
A microwave source for producing the microwave energy is further provided wherein the blowing device preferably blows the gas in a direction away from the microwave source. In one embodiment, the compressed gas has a pressure in the range of about 125 psi to 175 psi, and preferably about 175 psi.
In one embodiment, the photodetector is positioned on a bend in the waveguide run, which can be either pressurized or unpressurized. The bend can include 90 degree round bends, H-bends, and E-bends.
In one embodiment, the waveguide run carries the microwave energy to an oven cavity which has articles to be heated continuously fed therethrough. The oven cavity can also be heated by convection heating.
A method of suppressing an arc is also provided which includes providing microwave energy from a microwave source in a waveguide run, sensing an arc formed within the waveguide run, and blowing a gas into the waveguide run, in response to a sensed arc, for suppressing the arc. The method can also include the step of circularly polarizing the microwave energy.